This invention relates generally to the packaging of electronics, and more particularly, to the packaging of modules comprising silicon chips mounted on a substrate in a cap.
In one conventional packaging technique for silicon chips having integrated circuits thereon, the chips are first mounted on a ceramic substrate, and a sealant is applied over the chips causing the chips to be sealed onto the substrate. This constitutes what is known as a module. This module of one or more chips mounted on a substrate is then mounted in a cap with the chips facing inwardly. This mounting is accomplished by supporting the module in the cap, dispensing an epoxy onto the back surface of the module, allowing the epoxy to flow around the edges to seal the module into the cap, and thereafter curing the epoxy.
It has been found that a very desirable overcoat for covering the chips on the substrate is a silicone polymer. However, when utilizing this technique of mounting the modules onto caps with an epoxy when a silicone polymer has been used as a sealant around the chips, several rather serious problems have been encountered. One of these problems is characterized by bubbles having a tendency to form in the epoxy during conventional curing techniques. This formation of bubbles constitutes a product defect and requires the discarding of the assembled module and cap. It has been determined that the bubble formation is not caused by gas entrapped in the epoxy during dispense but rather is generated by the silicone polymer and this together with the air entrapped inside the cap during the curing cycle is forced into the epoxy and in many instances remains in the epoxy after it has completed its cure thus constituting a product defect.
Another problem which has been encountered when silicone polymer is used as an overcoat around the chips is that the epoxy during the curing cycle in some cases has been sucked inside the cap and in such instances will tend to displace the silicone polymer, with the epoxy working its way around the chip or chips. Subsequent heating and cooling cycles can cause the epoxy to expand and contract to such a degree that it will loosen the chips from the substrate. Another type of reliability defect encountered in the past due to "epoxy suck in" has to do with a "stacked" module. In this case the epoxy, when sucked in, forms a solid bridge between the two ceramic substrates and subsequent thermal cycling causes the epoxy to expand and contract thus causing the ceramic substrates to separate.